1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
Conventionally, as an example of an image forming apparatus, such as a copying machine or a laser beam printer, which forms an image using an electrophotographic method, a full-color image forming apparatus is known which is configured to superpose color component images of Y (yellow), M (magenta), C (cyan), and Bk (black) to form an image.
In addition, another conventional full-color image forming apparatus is known which includes a plurality of image bearing members and a plurality of development units and uses two types of toners, namely, a light color toner and a dark color toner having the same spectroscopic characteristics. In such a conventional full-color image forming apparatus, a light color toner is loaded in at least one of the plurality of development units, and a dark color toner is loaded in at least one of the remaining plurality of development units. In this regard, Japanese Patent Application Laid-Open No. 2000-231279 discusses an image forming method for expressing a color having one type of spectroscopic characteristic using a lookup table for light color toner and a lookup table for dark color toner, such as the ones illustrated in FIG. 3. In this method, a light color toner is preferentially used for a low density portion, a mixture of a light color toner and a dark color toner is used for a medium density portion, and a dark color toner is preferentially used for a high density portion. Thus, coarseness of dots in a low density portion can be made less conspicuous, and the amount of toner consumption for a high density portion can be reduced. Furthermore, granularity in a low density portion can be reduced. Thus, image quality can be improved and a color reproduction range can be enlarged.
A color image formed with color toners as described above may have glossiness different from that of the surface of a print paper sheet because the surface of such color image is smoothed during heat fixing. Moreover, since the viscosity of a toner varies during heat fixing depending on the type of a binder resin of a color toner or the type of heat fixing, the glossiness of a color image may vary.
Users may have different desires for glossiness of a color image according to the kind or intended use of an image. With respect to a photograph document with a picture of a person or scenery, an image having a high glossiness is usually desired from the viewpoint of obtaining a high quality image. In this regard, for example, Japanese Patent Application Laid-Open No. 05-142963 and Japanese Patent Application Laid-Open No. 03-2765 each discuss a method for selecting a material quality of toners and fixing conditions for toners in a color copying machine so as to form a high glossy image. However, with the method discussed in each of Japanese Patent Application Laid-Open No. 05-142963 and Japanese Patent Application Laid-Open No. 03-2765, although the glossiness of an image portion formed with toners can be made high, the glossiness of a non-image portion may not be made high. Accordingly, the glossiness on a transfer material may not be made uniform.
In this regard, Japanese Patent Application Laid-Open No. 63-58374 and Japanese Patent Application Laid-Open No. 04-278967 each discuss a method for transferring and fixing a transparent toner onto a transfer material, in addition to color toners. Moreover, Japanese Patent Application Laid-Open No. 63-58374 and Japanese Patent Application Laid-Open No. 04-278967 each discuss a method for transferring and fixing a white toner onto a non-image portion.
As development devices using color toners and a transparent toner, a two-component development device is widely used considering the stability of image quality. In such a full-color image forming apparatus, in order to stabilize image quality, a developer density ratio T/D (the weight ratio of a toner to a developer) is detected, and based on a result of the detection, a toner replenishment timing is determined. According to a conventional method, the developer density ratio T/D is detected using an automatic toner replenishment sensor (ATR sensor), such as an inductance sensor or a sensor for optically detecting the density of a developer, which is provided in a development device. In addition, various methods for detecting the developer density ratio T/D are known in which a latent image for a reference image is formed on a photosensitive drum serving as an image bearing member, the image density of a reference toner image (patch) is detected by a density sensor, and the developer density ratio T/D is determined based on the determined image density.
Japanese Patent Application Laid-Open No. 2006-47789 discusses an apparatus configured to use a light color toner development device and a dark color toner development device to form a toner image for detection of an image density so as to perform image control. This apparatus is configured to set the frequency of forming a toner image for detection of an image density in the dark color toner development device higher than in the light color toner development device. This is intended to more frequently perform correction control in the dark color toner development device, taking into consideration that the higher density of a dark color toner causes a greater density variation.
However, in a case where an image is formed using toners having mutually different color depths in the same hue, namely, using a light color toner development device and a dark color toner development device, the following problems arise. That is, an extreme variation in a color tint occurs in a density area mainly formed with a light color toner development device. In addition, in a case where an image is formed using a transparent tone development device and a dark color toner development device, the glossiness of a non-image portion formed with a transparent toner can be very unstable. These phenomena are described in detail below with respect to their causes.
As described above, in the case of a development using a light color toner and a dark color toner, an image is formed according to a lookup table such as the one illustrated in FIG. 3. At a commonly used average use density, for example, when an input image signal value is in a range of 100 to 140 (100 or greater and 140 or smaller), an output image signal value output from a light color toner development device is approximately in a range of 200 to 255 (200 or greater and 255 or smaller). That is, a development is performed at a high density equivalent to the density of a solid image area. On the other hand, an output image signal value output from a dark color toner development device is very low. More specifically, in forming an image having an average density, the amount of toner consumed in the light color toner development device is several times larger than the amount of toner consumed in the dark color toner development device.
In the case of a two-component development device, in order to stabilize the developer density, the amount of consumed toner is calculated using a detection unit to replenish a toner according to a result of the calculation. However, if an extreme amount of toner is consumed, the toner density can vary in a development device for the following reasons. This phenomenon is described in detail below.
As illustrated in FIG. 13, a two-component development device 132 stores therein a two-component developer including a nonmagnetic toner and a magnetic carrier. First and second stirring and carrying units 123 and 124 supply the developer to the surface of a development sleeve 121, which serves as a developer bearing member. The developer is retained on the surface of the development sleeve 121 in the form of a magnetic brush with a magnetic force from a magnet roller, which serves as a magnetic field generation unit, in the development sleeve 121. The retained developer is carried to a portion facing a photosensitive drum 101 (development area) according to the rotation of the development sleeve 121. A developer return member 126 and an ear height restriction member 127 cut an ear of the magnetic brush of the developer to properly maintain the amount of a developer carried to the development area.
The inside of the development device 132 is partitioned into a development chamber (a first chamber) 129 and a stirring chamber (a second chamber) 130 with a partition wall 128 extending in a direction perpendicular to the drawing surface of FIG. 13. The first and second developer stirring and carrying units 123 and 124 are disposed in the development chamber 129 and the stirring chamber 130, respectively.
The first stirring and carrying unit 123 is disposed substantially in parallel to an axial direction of the development sleeve 121 below the development chamber 129. The first stirring and carrying unit 123 has a screw structure having blade members around a rotation shaft thereof in a spiral-like form. The first stirring and carrying unit 123 rotates to carry a developer in the development chamber 129 in one direction along the axis of the development sleeve 121.
The second stirring and carrying unit 124 has a screw structure similar to that of the first stirring and carrying unit 123. However, the second stirring and carrying unit 124 has blade members around a rotation shaft thereof in a spiral-like manner having an opposite direction to that of the first stirring and carrying unit 123. The second stirring and carrying unit 124 is disposed substantially in parallel to the first stirring and carrying unit 123 below the stirring chamber 130. The second stirring and carrying unit 124 rotates in the same direction as the stirring and carrying unit 123 to carry a developer in the stirring chamber 130 in a direction opposite to that of the stirring and carrying unit 123.
At front and back edge portions of the partition wall 128, developer paths 128a and 128b are provided, which mutually connect the development chamber 129 and the stirring chamber 130, as illustrated in FIG. 14. As the developer is carried by the stirring and carrying units 123 and 124, the developer in the development chamber 129 flows into the stirring chamber 130 via the developer path 128 of the partition wall 128. The developer in the stirring chamber 130 flows into the development chamber 129 via the other developer path 128 of the partition wall 128. Thus, the developer circulates between the development chamber 129 and the stirring chamber 130.
The development device 132 includes a toner replenishment tank (not shown). A toner is supplied into the stirring chamber 130 from the toner replenishment tank at a position upstream of the second stirring and carrying unit 124 under the control of a developer density control device (not shown). The developer inside the development chamber 129, whose toner density is lowered due to consumption of toner by development, is supplied into the stirring chamber 130 by the first stirring and carrying unit 123. Then, the developer is stirred and mixed by the second stirring and carrying unit 124 with the developer already existing in the development chamber 129 and the toner supplied from the toner replenishment tank to uniform the toner density of the developer. Subsequently, the developer is carried into the development chamber 129.
However, in this case, if an extremely large amount of toner is consumed, the amount of toner consumed on the development sleeve 121 naturally becomes large. Accordingly, the developer in the development chamber 129 is supplied into the stirring chamber 130 by the first stirring and carrying unit 123 in a state in which the toner density of the developer is extremely lowered due to an extreme consumption of toner. Accordingly, even if the developer is stirred and mixed by the second stirring and carrying unit 124, the toner density of the developer cannot be easily made uniform. Thus, the toner density in the development device 132 may become nonuniform. When the toner density varies, the amount of toner charge also varies. As a result, the image density also varies. Accordingly, depending on the circulation of the developer in the development device 132, in the case where the amount of toner consumption is extremely large, the effect of uniforming the toner density in the development device 132 cannot be sufficient. Thus, the toner density in the development device 132 may not be made uniform. As a result, the image density may vary. Accordingly, the quality of images degrades with a different hue for each image.
In this regard, the amount of toner consumed in a light color toner development device can be reduced by reducing the amount of toner necessary for development using a light color toner at an average density value while lowering the output level of a medium image signal for a light color toner in a light color toner lookup table. In this case, however, an advantage of using a light color toner for reducing granularity in a low-density portion can be lost.
In addition, a transparent toner is used to alleviate unevenness in an image surface by forming an image in an area in which a color toner does not exist. Accordingly, in the case of development using a transparent toner, depending on its state of use, an amount of development toner equal to or larger than an amount of toner formed with a plurality of color toners is necessary. Thus, using a transparent toner creates a problem similar to the problems in the case of development using a light color toner.